Praise be to Allah.
What he must do is adhere to the truth which is indicated by the Book of Allaah and the Sunnah of the Messenger (blessings and peace of Allaah be upon him), and his friendship and enmity must be based on that. Any party or group that goes against the truth, he must disavow himself thereof and not agree with it. The religion of Allaah is one: it is the straight path, it is worship of Allaah alone and following His Messenger Muhammad (blessings and peace of Allaah be upon him).
What every Muslim must do is adhere to this truth and follow it; it is obedience to Allaah and following His sharee’ah which was brought by His Prophet Muhammad (blessings and peace of Allaah be upon him), being utterly sincere to Allaah in doing so and not devoting any act of worship to anyone other than Him, may He be glorified and exalted. Every group that goes against this and every party that does not adhere to this belief, he must distance himself therefrom and disavow himself of it and call its people to the truth on the basis of shar’i evidence and with kindness, and he should seek out useful means of doing so to show the truth to them clearly. End quote.